Enabling coordinated multi-point reception

ABSTRACT

This invention measures the propagation delay τ 1  between the user equipment and a first cooperating unit and the propagation delay τ 2  between the user equipment and a second cooperating unit. These propagation delays are used to compute a timing advance amount to the user equipment to enable coordinated multi-point reception. In a first embodiment one cooperating unit receives a function of the propagation delay, computes the timing advance amount and transmits a timing advance command to the user equipment. In a second embodiment a central unit performs these operations.

CLAIM OF PRIORITY

This application is a continuation of prior application Ser. No.12/611,547, filed Nov. 3, 2009, which claims priority under 35 U.S.C.119(e)(1) to U.S. Provisional Application No. 61/110,685 filed Nov. 3,2008.

TECHNICAL FIELD OF THE INVENTION

The technical field of this invention is protocols in wirelesscommunications.

BACKGROUND OF THE INVENTION

FIG. 1 shows an exemplary wireless telecommunications network 100. Theillustrative telecommunications network includes base stations 101, 102and 103, though in operation, a telecommunications network necessarilyincludes many more base stations. Each of base stations 101, 102 and 103are operable over corresponding coverage areas 104, 105 and 106. Eachbase station's coverage area is further divided into cells. In theillustrated network, each base station's coverage area is divided intothree cells. Handset or other user equipment (UE) 109 is shown in Cell A108. Cell A 108 is within coverage area 104 of base station 101. Basestation 101 transmits to and receives transmissions from UE 109. As UE109 moves out of Cell A 108 and into Cell B 107, UE 109 may be handedover to base station 102. Because UE 109 is synchronized with basestation 101, UE 109 can employ non-synchronized random access toinitiate handover to base station 102.

A number of advanced features are being considered to enhance the cellthroughput and/or the cell-edge throughput of the Evolved UniversalTerrestrial Radio Access (E-UTRA) during the long term evolution (LTE)study stage. In coordinated multi-point reception (also know asmacro-diversity), received signals from multiple cooperating units arecombined and processed at a single location. This single location may bedifferent for different user equipments (UEs). In different instances,cooperating units can be separate e-NodeBs, remote-radio units (RRUs),relays etc. The main concern is that coordinated multi-point receptionrequires coordinated multi-point synchronization.

Coordinated multi-point reception has a problem in different signalpropagation delays from the UE to different cooperating units. FIG. 1illustrates a first propagation delay τ₁ between UE 201 and cooperatingunit 211 and a second propagation delay τ₂ between UE 201 andcooperating unit 212.

Proper receiver operation in any communication system requiresappropriate timing synchronization with the transmitter. Multi-pointreception requires the UE to be simultaneously synchronized to bothcooperating unit receivers. In certain cases, this would be practicallyimpossible to achieve. Whenever |τ₁-τ₂| exceeds the cyclic prefixduration this goal is practically impossible to achieve. However, it ispossible to achieve simultaneous synchronization to differentcooperating units in most other cases.

As in any other Orthogonal Frequency Division Multiplexing (OFDM) basedsystem, the receiver timing can be regarded as a reference point forsynchronization. All UEs talking to the receiver should adjust theirtransmission timing so that their signals arrive approximatelysimultaneously within the Cyclic Prefix (CP) tolerance at the receiver.Timing advance (TA) commands are sent to each UE to compensate for thepropagation delay in the channel. Upon reception of a timing advancecommand, the UE adjusts its uplink transmission timing for PhysicalUplink Control CHannel (PUCCH), Physical Uplink Shared CHannel (PUCCH)and sounding reference signals (SRS). The timing advance commandindicates the change of uplink timing relative to the current uplinktiming in multiples of a frame time constant. This propagation delay iscommonly understood to be the first arriving path. For the exampleillustrated in FIG. 2, a TA command to advance timing, with respect tocooperating unit 211, by τ₁, could be transmitted to the UE 201. Thenthe CP removal absorbs all the trailing paths from UE 201 to cooperatingunit 211. In certain implementations the UE is expected to be heard bycooperating receivers at multiple different cooperating units, withdifferent propagation delays. This situation is illustrated in FIG. 1where UE 201 transmits to both cooperating receivers 211 and 211.Consequently, TA commands ought to depend on propagation delays to mostor all of the cooperating units.

SUMMARY OF THE INVENTION

This invention concerns overcoming some issues with coordinatedmulti-point reception. This invention also presents an initial study ofthroughput enhancements through Macro-diversity via cooperatingreceiving units.

In this invention at least two cooperating base station units measurethe propagation delay τ₁ between the user equipment and a firstcooperating unit and the propagation delay τ₂ between the user equipmentand a second cooperating unit. These propagation delays are used tocompute a timing advance amount to the user equipment to enablecoordinated multi-point reception. In a first embodiment the secondcooperating unit computes function f(τ₂) of the propagation delay τ₂which it transmits to the first cooperating unit. The first cooperatingunit computes the timing advance amount and transmits a correspondingtiming advance command to the user equipment. In a second embodiment acentral unit receives respective functions f(τ₁) and f(τ₂) and computesthe timing advance amount. This central unit them transmits thecorresponding timing advance command to the user equipment. The functionf(τ) may be quantization of the propagation delay τ or a suggestedtiming advance amount.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of this invention are illustrated in thedrawings, in which:

FIG. 1 is a diagram of a communication system of the prior art relatedto this invention having three cells;

FIG. 2 illustrates the typical signal propagation delays at cooperatingunits in the prior art;

FIG. 3 illustrates computing a timing advance command at one cooperatingunit according to this invention;

FIG. 4 is a flow chart illustrating the steps of the embodiment of theinvention illustrated in FIG. 3;

FIG. 5 illustrates computing a timing advance command by a networkaccording to this invention;

FIG. 6 is a flow chart illustrating the steps of the embodiment of theinvention illustrated in FIG. 6;

FIG. 7 illustrates simulated results of the cell edge throughput versuscell throughput for full path loss compensation according to thisinvention; and

FIG. 8 illustrates simulated results of the cell edge throughput versuscell throughput for partial path loss compensation according to thisinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3 illustrates a first embodiment for achieving multi-pointsynchronization uses the concept of a serving/primary cooperating unit.This serving/primary cooperating unit may be an e-NodeB. The non-servingor second cooperating unit still participates in signal detection forthe UE. Second cooperating unit 212 measures the propagation delay τ₂from UE 201 to itself. Second cooperating unit 221 then transmits f(τ₂),which is a function of this propagation delay τ₂, to first cooperatingunit 211. Function f(τ₂) could be either a quantization of the measuredpropagation delay τ₂ or a TA suggestion by second cooperating unit 212.First cooperating unit 211 then processes all the needed information anddecides on the TA command to be sent to UE 210. First cooperating unit211 then transmits the TA command to UE 210 including g(f(τ₂), τ₁),which is a function of f(τ₂) and τ₁. These transmissions are illustratedin FIG. 2. Mechanisms for these transmissions are already existentincluding: Random Access Channel (RACH); and sounding reference signals(SRS). The network determines whether UE 201 transmits via RACH or SRSto second cooperating unit 212.

FIG. 4 illustrate steps of the method 400 of the embodiment illustratedin FIG. 3. Method 400 begins at start block 401. At block 402cooperating unit 211 measures the propagation delay τ₁ from UE 201. Atblock 403 cooperating unit 212 measures the propagation delay τ₂ from UE202. As shown in the parallel structure in FIG. 4 these measurements areindependent and can occur sequentially or simultaneously.

In block 404 second cooperating unit 212 computes the function f(τ₂). Asnoted above function f(τ₂) could be either a quantization of themeasured propagation delay τ₂ or a TA amount suggestion. In block 405second cooperating unit 212 transmits this function f(τ₂) to firstcooperating unit 211.

In block 406 first cooperating unit 211 computes the function g(f(τ₂),τ₁). In block 407 first cooperating unit 211 transmits a TA command toUE 201 corresponding to the computed function g(f(τ₂), τ₁). This TAcommand is selected to enable coordinated multi-point reception at UE201. In block 408 UE 201 operates according to this TA command.

The technique described in relation to FIGS. 3 and 4 is appropriate forthe case where cooperating units are e-NodeBs. However, thisfunctionality may also be built into more advanced remote radio units.In general, a measurement based on signal time delay could be defined bythe measurement specification. The signal time delay is expected to bespecific to a particular UE. This measurement can be communicated eitherto the network or directly to another cooperating unit.

A second embodiment to achieve network synchronization is a symmetricoption illustrated in FIG. 5. Each cooperating unit 211 and 212determines the corresponding propagation delay τ₁ and τ₂ to UE 201.Cooperating units 211 and 212 make corresponding inferences f(τ₁) andf(τ₂) on the respective propagation delays. Cooperating units 211 and212 transmit the inferences f(τ₁) and f(τ₂) to central location 220.Central location 220 computes the needed TA command and sends it to UE201. Note that the TA command can also be sent through one or bothcooperating units 211 or 212.

FIG. 6 illustrate steps of the method 600 of the embodiment illustratedin FIG. 5. Method 600 begins at start block 601. At block 602cooperating unit 211 measures the propagation delay τ₁ from UE 201. Atblock 603 cooperating unit 212 measures the propagation delay τ₂ from UE202. In block 604 first cooperating unit 211 computes the functionf(τ₁). In block 605 second cooperating unit 212 computes the functionf(τ₂). In block 606 first cooperating unit 211 transmits the functionf(τ₁) to central unit 220. In block 607 second cooperating unit 212transmits the function f(τ₂) to central unit 220. As shown in theparallel structure in FIG. 6 the two chains 602-604-606 and 603-605-607are independent and can occur sequentially or simultaneously.

In block 608 central unit 220 computes the function amount needed in aTA command to UE 201. This TA command amount is selected to enablecoordinated multi-point reception at UE 201. In block 609 central unit220 transmits a TA command to UE 201 corresponding to the computed TAamount. In block 610 UE 201 operates according to this TA command.

In general, when Macro-diversity is used in the uplink (UL) with signalsprocessed at a central location, the system capacity can be extremelylarge. A portion of the network can be treated a single very largeMultiuser, Multiple Input, Multiple Output (MU-MIMO) system becomingalmost purely Adaptive White Gaussian Noise (AWGN) limited. However, thecomputational complexity of this technique could become very large,especially when using more advanced receiver techniques.

The following is an example of how the network computes the timingadvance. In FIG. 3 cooperating unit 211 is the serving cell for UE 201.Cooperating unit 211 measures the propagation delay τ₁ using randomaccess preamble transmission from UE 201 or SRS transmission from UE201. Cooperating unit 212 provides a SRS sequence or RACH preamble to UE201 via the serving cell 211. UE 201 is instructed at regular intervalsto transmit the SRS or RACH preamble to cooperating unit 212 enablingcooperating unit 212 to measure the propagation delay τ₂. The functionf(τ₂) could be the absolute propagation delay τ₂ or a delta offset valuefrom the common network timing if both cooperating units aresynchronized. The serving cell, cooperating unit 211, then computes thetiming advance command g(f(τ₂), τ₁) such that |τ₁-τ₂| is within atolerable fraction of the cyclic prefix of each cooperating unit.

This scheme is generally applicable to K cooperating units, with Kgreater than or equal to 2. The set of cooperating units for a UE can bechosen based on multiple criteria such as reference signal receivedpower (RSRP), signal to interference plus noise ratio (SINR) andpropagation delay. An example is as follows. The UE sequentiallytransmits SRS to K base stations, where the RSRP of the Kth base stationis within a defined tolerance of the RSRP of the serving base station ofthe UE. Alternatively, the UE sequentially transmits SRS to K basestations where the uplink SINR seen by the Kth base station is within adefined tolerance of the SINR seen by the serving base station of theUE. The Kth base station measures the propagation delay τ_(k) and sendsf(τ_(k)) to a network decision unit. The network decision unitdetermines a set of M cooperating units from the K (M≦K) base stationssuch that for the Mth base station |τ₀-τ_(m)| is within a tolerablefraction of its cyclic prefix, where τ₀ is a network reference time. Thenetwork then sends the appropriate timing advance command to the UE.FIG. 5 illustrates this example for M=2 cooperating units 211 and 212,one UE 201 and network decision unit 220.

The following describes a simulation of the invention in a 19-sitesystem having a matched filter receiver combining signals acrosscellular sites and different cells. This effectively combines the signalto interference plus noise ratios (SINRs) of any given user at multiplecellular sites. Table 1 lists the assumptions employed in thissimulation.

TABLE 1 Parameter Assumption Cellular Layout Hexagonal Grid; 19 NodeBsUser Drop Uniformly Inside the Cell Minimum Distance Between UE and 35 mNodeB Antenna Bore Site Towards Flat Side of the Cell Inter - SiteDistance 500 m or 1732 m Shadowing Standard Deviation 8 dB Path Loss128.1 + 37.6log₁₀ (R) where R is Shadowing Standard Deviation 8 dBShadowing Between Cells 1.0 Correlation Between NodeBs 0.5 PenetrationLoss 2 dB Antenna Pattern A = − min {12 (θ/θ_(3dB))², 20 dB}. SystemBandwidth 2.5 MHz @ 2 GHz Numerology RB size 24 Sub-Carriers Number ofRBs 6 Channel Model SCM-C UE Velocity kmh or 30 kmh UE Power Class 24dBm Number of UE Antennas 1 Number of NodeB Antennas 2 ChannelEstimation Penalty 1 dB UE Antenna Gain 0 dBi NodeB Antenna Gain 14 dBiNumber of UEs per NodeB/Cell 18/6 HARQ Type Chase Combining MaximumNumber of 5 Retransmissions HARQ Retransmission Delay 5 TTI TrafficModel Full Buffer Scheduler Proportional Fair Scheduling Delay 1 TTIUplink Power Control Slow with 40 TTI Period MCS Set QPSK: {1/5, 1/4,1/3, 1/2, 5/8} 16QAM: {1/3, 1/2, 5/8, 3/4}Table 2 lists the UE power settings examined in this simulation.

TABLE 2 System Configuration 1 2 3 4 5 % of UEs at Pmax 1% 5% 10% 25%50%It is possible to further enhance the system throughput using a minimummean square error (MMSE) receiver.

FIGS. 7 and 8 illustrate simulation results. FIG. 7 illustrates thesimulated throughput results under the full path loss compensation. FIG.7 includes points for each of the system power configurations listed inTable 2 for one cell processing and for multi-cell processing. Asillustrated in FIG. 7, the gain of macro-diversity SINR combining ismostly to enhance cell-edge performance. For example, for a cellthroughput of 0.77 b/s/Hz/cell, one cell processing provides about 0.15b/s/Hz/cell for cell edge whereas multi-cell processing gives above 0.5b/s/Hz/cell. Thus, the cell-edge throughput can be substantiallyenhanced using macro-diversity. FIG. 8 illustrates a similar result forsystem throughputs with partial path loss compensation.

Even using simple SINR combining at a central processing location, thecell-edge throughput can be substantially enhanced. Depending on theoperating point of cell-average throughput, this enhancement can be afactor of a few to several. Furthermore, using more advanced MIMOprocessing at the receiver further improves the extent of potentialgains. Multi-point synchronization Therefore thus enable macro-diversityreception.

What is claimed is:
 1. A method of operating a base station, comprising: measuring a propagation delay τ₁ between a user equipment and the base station; computing a function g(f(τ₂), τ₁) using a function f(τ₂) of another delay τ₂ and the propagation delay τ₁; computing a timing advance amount for the user equipment from the computed function g(f(τ₂), τ₁); and transmitting a timing advance command having the computed timing advance amount to the user equipment.
 2. The method of claim 1, wherein another delay τ₂ is a propagation delay.
 3. The method of claim 2, wherein: said function f(τ₂) is a quantization of the propagation delay τ₂.
 4. The method of claim 2, wherein: said function f(τ₂) is a suggested timing advance amount corresponding to the propagation delay τ₂.
 5. The method of claim 1, wherein said another delay τ₂ is measured by another base station.
 6. The method of claim 1, wherein said another delay τ₂ is the delay between the user equipment and another base station.
 7. The method of claim 1, wherein said function f(τ₂) is calculated in another base station.
 8. The method of claim 6, said function f(τ₂) is wirelessly transmitted to said base station.
 9. The method of claim 1, wherein the computed function g(f(τ₂), τ₁) used in coordinated multi-point reception.
 10. The method of claim 1, wherein said propagation delay τ₁ is measured at the base station.
 11. The method ref claim 1, wherein said function g(f(τ₂), τ₁) reputed at the base station.
 12. The method of claim 1, wherein the base station transmits said timing advance command having the computed timing advance amount to the user equipment.
 13. A method of operating a transceiver, comprising: measuring a propagation delay τ₁ between a first transceiver and a second transceiver; computing a function g(τ₂), τ₁) using a function f(τ₂) of another delay τ₂ and the propagation delay τ₁; computing a timing advance amount for the first transceiver from the computed function g(f(τ₂), τ₁); and transmitting a timing advance command having the computed timing advance amount to the first transceiver.
 14. The method of claim 13, wherein another delay τ₂ is a propagation delay.
 15. The method of claim 14, wherein: said function f(τ₂) is a quantization of the propagation delay τ₂.
 16. The method of claim 14, wherein: said function f(τ₂) is a suggested timing advance amount corresponding to the propagation delay τ₂.
 17. The method of claim 13, wherein said another delay τ₂ is measured by a third transceiver.
 18. The method of claim 13, wherein said another delay τ₂ is the delay between the first transceiver a third transceiver.
 19. The method of claim 13, wherein said function f(τ₂) calculated in a third transceiver.
 20. The method of claim 19, said function f(τ₂) is wirelessly transmitted to said second transceiver.
 21. The method of claim 13, wherein the computed function g(f(τ₂), τ₁) is used in coordinated multi-point reception.
 22. The method of claim 13, wherein said propagation delay τ₁ is measured at the second transceiver.
 23. The method claim 13, wherein said function g(f(τ₂), τ₁) is computed at the second transceiver.
 24. The method of claim 13, wherein the second transceiver transmits said timing advance command having the computed timing advance amount to the first transceiver.
 25. A method of operating a transceiver, comprising: receiving a timing advance command having a computed timing advance amount, the computed timing advance amount computed from a function g(f(τ₂), τ₁) computed using a function f(τ₂) of another delay τ₂ and a propagation delay τ₁ between the transceiver and a base station; and operating the transceiver according to the timing advance command. 